The accident and emergency department at University College Hospital Galway is to operate at a reduced capacity until the new year because of refurbishment works that began on Monday.
Serious concerns have been expressed at the timing of the works, which are taking place during what is normally a peak period for admissions.
A section of the department will have to be closed off. The work is to include the provision of new treatment and plaster rooms and a new minor injuries area.
A spokesperson for the Health Service Executive (West) said contingency arrangements were being put in place.
"We would also ask members of the public for their support during this period and patients to attend their general practitioner where possible."
The HSE said the floor space of the A&E department would be reduced by approximately 30 per cent during the works.
Industrial relations officer with the Irish Nurses' Organisation Noreen Muldoon said A&E staff endured one of their worst ever pressure days on Monday, with a record 110 patients attending and 20 waiting overnight for attention.
"We know this refurbishment work is necessary and it will lead to an improvement in facilities, but measures must be put in place today to deal with the situation on the ground at the A&E department," she said.
The reduction of about 30 per cent of casualty space and the closing off of a corridor that could help take overflow cases, combined with the record numbers presenting for treatment, have been cited by nurses as the main causes of the problems.
West Galway Fine Gael TD Padraic McCormack said there were concerns among the public over the timing of the works and the availability of alternative beds in different parts of the hospital.
"For the past two to three years there is on average 60 beds unused at UCHG because the staff aren't being provided by the Minister for Health.
"I am this week calling on the Minister for Health to put the staff in place to allow for the opening of these beds on a permanent basis, which would ensure that there would be no reduction of capacity in the A&E section," he said.
